An existing current to voltage converting module, i.e. an I-V converting module, as shown in FIG. 1, consists of a regular I-V transforming circuit 1, a sampling and holding circuit 2, and a current output sensor 3. The sampling and holding circuit 2 counteracts a direct current component output by the current output sensor 3 to enlarge an output dynamic range of the I-V converting module. However, a setup speed of the existing sampling and holding circuit 2 is relatively slow. This decreases a speed of the entire I-V conversion, in other words, this results in a longer time for obtaining an alternating current signal.
In the existing I-V converting module, if a current output by the current output sensor 3 is I0, and the direct current component output by the current output sensor 3 is I, in a sampling setup stage, that is, when a control signal sh is at a high level, a switch S2 is closed. In this situation, a P-type field-effect transistor M2 is equivalent to a resistor having a resistance of gm. Because a field-effect transistor M2 usually works in a sub-threshold region, the resistance of the field-effect transistor M2 is in positive proportion to the direct current component I output by the current output sensor 3, that is, gm=α·I (α is a constant). When the control signal sh is at a low level, that is, a moment when the switch S2 is open, a voltage of a plate of a parasitic capacitor C3 connected to a sampling capacitor C2 begins to be established, and a time constant of the sampling and holding circuit 2 is therefore τ=(C3+C2)/gm=(C3+C2)/α·I.
Taking a parameter that frequently appears in the above formula as an example, if gm=α·I=1 μS, C3=100 pF, and C2=1 pF, the time constant is τ=100 μs. During the conception of the present invention, the inventor discovered that the existing technology has the following problems: the setup speed of the sampling and holding circuit 2 is very low, and consequently the I-V transforming circuit 1 needs a long time to collect an alternating current component output by the current output sensor 3 and the converting rate of the I-V converting module may be severely decreased.